Rongeurs are surgical instruments designed to bite off and hold fragments of tissue. They have jaws, in the form of a cup, that can be opened and closed. They are made in a vast variety of sizes and shapes consistent with their intended purposes. For example, those designed to remove bone are generally quite strong and deliberately designed so that the biting area times the force generated by that biting is considerably less than the supporting area and the ability of the supporting structures to withstand such forces without damage. Other rongeurs, such as those used in operative arthroscopy (surgery performed on a joint through a small puncture using an illuminated telescope like device about the size of a pencil and various instruments inserted through a similar puncture) or an operative endoscopy (same as above but anywhere within the body rather than just a joint), are designed to only bite softer tissues and may be significantly constrained in their design and dimensions by their intended site of use. For example, those rongeurs used to remove tissue from a temporomandibular joint (hinge joint of the jaw) must, because of the small size of the joint, be rather small and correspondingly delicate.
However, as a practical matter for any given hardness of tissue to be bitten, there is a minimum thickness needed to support the biting edge regardless of the size of the bite. Since these rongeurs bite around a piece of tissue using their sharpened perimeters and do not punch out the tissue over the entire surface area, then the thickness and strength of the cutting edge and supporting infrastructure is dictated by the hardness and thickness of the tissue to be cut. Therefore, the first problem that arises with the design of a rongeur is that as the tip of the instrument gets smaller, the needed edge wall thickness decreases at a lesser rate such that one would end up with a set of cutting edges that were so close together that there would be virtually no area in between to be cut. Furthermore, while that would already be undesirable, to make an instrument small enough for the purpose intended, even a rongeur reduced to only biting edges and with no central area might actually be still too large if one attempted to preserve the optimal edge thickness in regards to the tissue to be removed. As a practical matter therefore, small rongeurs such as those used to perform operative arthroscopy are limited to the purpose of removing soft tissues only.
When an unexpectedly hard piece of tissue or bone is accidentally bitten during the surgery, then the instrument may be damaged or destroyed by a failure at either the tip or at either the proximal or distal axis pins.
At present, in an attempt to protect the delicate working tips of these rongeurs, some rongeurs have been constructed so that some other area than the tip will fail when overloaded, thus preserving the integrity of the tip. This would appear to be logical as the entire instrument only exists to make the tip work and as a practical matter when the tips are damaged the instrument is generally not repairable. The most likely candidate to be designed for first failure would then seem to be the distal pin. That is because empirically these structures already have a high failure rate and because of their requisite small size, dictated by the small dimensions of the working tip, they are rather weak to begin with. If the tip is protected by failure of the distal pin, then the instrument can be repaired by pushing out the distal pin and replacing it with a new pin, a procedure that can be performed at a factory repair center. However, this is unacceptable as a failure of the distal pin occurring inside the human body and particularly within a joint might result in loose steel fragments within that joint which could severely damage the joint or result in further or additional surgery to remove such fragments. Therefore, most instrument makers have elected to assure that any instrument failure occurs outside the joint and have done so by markedly weakening the proximal pin or equivalent mechanism. Unfortunately, to be reasonably assured that this will indeed be the point of failure, it is necessary to make the proximal pin considerably weaker than the already very weak (by the necessity of the size limitations) distal pin resulting in a significantly compromised and weak instrument that fails easily and must be returned to the factory service center repeatedly for repair at a not insignificant cost.
One approach employed to prevent the damage to the tip has involved the insertion of a spring relief mechanism interposed between the handle and the tip sold under the Dyonics trademark. While such a spring mechanism does prevent the breaking of the tip, as shown in FIGS. 7 and 8, the entire tip moves forward upon application of excessive pressure. The movement of the tip results in the undesirable problem of the user either balancing the movement of the tip by withdrawing it slightly, or compensating for the forward movement during the grasping. Either of these approaches are undesirable. In addition, the above prior device was complicated, difficult and expensive to manufacture.